Typically, a power converter is used for converting a power into a necessary or desirable power output tailored for a specific application. However, when the power supply that is designated to provide input power to the power converter is first switched on, a surge input current (known as inrush current) may be generated which is many times greater than the steady state current until the power supply reaches equilibrium. This abnormal inrush current will charge the smoothing capacitor at the output end of the power converter and in turn impair the diodes of a bridge rectifier.
To combat the inrush current problem, a resistor element may be utilized as an inrush current protection circuit. In the conventional art, a thermistor having a positive/negative temperature coefficient characteristic is employed as a current limiter for limiting inrush current from increasing. An example of such an inrush current protection circuit is given in U.S. Pat. No. 5,995,392, which is incorporated herein by reference. In this prior art reference, a current limiter including a fixed resistor and a positive temperature coefficient (PTC) thermistor that are connected in series across a control switch. The current limiter disclosed in this example is used to limit the inrush current supplied to the rectifier input terminal. In case of an excessive inrush current being applied to the power converter, the current flowing through the PTC thermistor causes it to heat up, and its resistance increases. The change in the resistance of the PTC thermistor is sufficiently high to decrease the current being drawn through the resistors to a safe value. Therefore, the remainder of the power converter is isolated from the input voltage, and thereby prevents the flickering of the power supply.
In addition to the inrush current protection circuit, it is necessary to add a polarity reversal protection circuit to ensure correct polarity connection of the power converter to the power supply. As can be known by one skilled in the art, if the power converter is connected to a power supply with reverse polarity, the power converter maybe inoperative, and may cause permanent damage or destruction to the electronic circuit. The polarity reversal protection circuit is then used as a failsafe device to avoid damages to the internal circuit of the power converter in case of polarity reversal. In most practical applications, the straightforward method for protecting a power converter from reversed supply voltage is to place a diode between the input terminal of power supply and the internal circuit of the power converter. The diode will allow a current to flow from the power supply to the internal circuit of the power converter when the diode is forward biased from a correct polarized supply voltage, and disallow a current to flow from the power supply to the internal circuit of the power converter when the power supply is connected in reverse direction. An embodiment of such prior polarity reversal protection circuit is explicated in U.S. Pat. No. 6,304,422, which is also incorporated herein by reference.
Although the prior art polarity reversal protection diode as described above can substantially protect the power converter from possible damages due to reverse polarity connection, it is a problem that the forward voltage drop appearing on polarity reversal protection diode when it is forward biased will cause a significant power loss during power conversion process. As a result of the power loss resulting from the voltage drop on the diode, the overall power conversion efficient of the power converter will be degraded, and the noise immunity of electronic circuit is deteriorated.
There is, therefore, an inclination to combine the polarity reversal protection circuit and inrush current protection circuit into a single power converter with fewer circuit components and reduced power loss.